Limiting peak electrical power drawn by mining excavators

ABSTRACT

The maximum electrical power drawn from an electrical power source by a mining excavator comprising electric motors is reduced by supplying supplementary electrical power from an electrical energy storage unit. The input electrical power drawn by the mining excavator is cyclic. An upper limit is set for the electrical power drawn from the electrical power source. When the input electrical power drawn by the mining excavator exceeds the upper limit, electrical power is supplied by the electrical energy storage unit, such as an ultracapacitor bank. The ultracapacitor bank may be charged by the electrical power source during off-peak intervals. Electrical power generated by electrical motors operating in a regeneration interval may also be recaptured and stored in the electrical energy storage unit.

BACKGROUND OF THE INVENTION

The present invention relates generally to electrical power systems, andparticularly to systems for limiting peak electrical power drawn bymining excavators from an electrical power source.

Many applications depend on electricity supplied by an electrical powerdistribution network, such as the electrical power grid operated by anelectrical power utility company. Some commercial and industrialapplications draw significant power. Loads are often dynamic, and peakpower demand may approach, and, in some instances, exceed, the maximumpower available from the electrical power distribution network.Excessive peak power demand may lead to voltage sags and temporaryoutages in the electrical power distribution network. Therefore, notonly may the performance and reliability of the application of interestbe degraded, but also service to other customers of the electrical powerutility company may be disrupted.

One application which draws significant electrical power is mining. In amining operation, the electrical power distribution network feeds a widespectrum of loads, ranging from small industrial motors to largedraglines. Electrical mining excavators, such as electric shovels anddraglines, present a cyclic load to the electrical power distributionnetwork. Although the average power drawn by these machines may be about55% of their peak power demand, in some instances, the peak power demandmay approach the generation limits at the individual feeders providinginput power to the machines. For example, an electric shovel may drawpeak powers on the order of 3.5 megawatts, and a dragline may draw peakpowers on the order of 24 megawatts.

In addition to improved performance and reliability, there is also aneconomic incentive for reducing peak power demand. Electrical powerutility companies supplying power to the mines typically measure thepower demand of a mine based on 15-minute intervals, and billing isadjusted for peak power demand during each 15-minute interval. What areneeded are method and apparatus for limiting peak power drawn by miningexcavators from an electrical power distribution network. Method andapparatus which reduce wasted energy are particularly advantageous.

BRIEF SUMMARY OF THE INVENTION

In an embodiment of the invention, the maximum electrical power drawnfrom an electrical power source by a mining excavator comprisingelectric motors is reduced by supplying electrical power from anelectrical energy storage unit. The electrical power drawn by the miningexcavator is cyclic. An upper limit is set for the electrical powerdrawn from the electrical power source. When the input power drawn bythe mining excavator exceeds the upper limit, additional electricalpower is supplied by the electrical energy storage unit.

One embodiment of an electrical energy storage unit is an ultracapacitorbank, which may be charged by the electrical power source when theelectrical power drawn by the mining excavator is less than the upperlimit. In another embodiment of the invention, electrical powergenerated by electrical motors operating in a regeneration interval isstored in the electrical energy storage unit.

These and other advantages of the invention will be apparent to those ofordinary skill in the art by reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a high-level schematic of an electric shovel;

FIG. 2A shows a high-level schematic of a cyclic load drawing electricalpower from an electrical power source;

FIG. 2B shows a plot of the power demand of a cyclic load with noregeneration;

FIG. 2C shows a plot of the output power drawn from the electrical powersource shown in FIG. 2A corresponding to the plot of power demand shownin FIG. 2B;

FIG. 2D shows a plot of the power demand of a cyclic load withregeneration;

FIG. 2E shows a plot of the output power drawn from the electrical powersource in FIG. 2A corresponding to the plot of power demand shown inFIG. 2D;

FIG. 3A shows a high-level schematic of a cyclic load drawing electricalpower from an electrical power source and an electrical energy storageunit;

FIG. 3B shows a plot of the power demand of a cyclic load withregeneration;

FIG. 3C shows a plot of the output power drawn from the electrical powersource shown in FIG. 3A corresponding to the plot of power demand shownin FIG. 3B;

FIG. 3D shows a plot of the power demand of a cyclic load with noregeneration;

FIG. 3E shows a plot of the output power drawn from the electrical powersource in FIG. 3A corresponding to the plot of power demand shown inFIG. 3D;

FIG. 4 shows a single-line diagram of an electric shovel control system;

FIG. 5 shows a schematic of an electrical power system with anintegrated ultracapacitor bank;

FIG. 6 shows a plot of the power demand of an electric shovel;

FIG. 7 shows a plot of the output power drawn from an electrical powersource when the output power is constrained between an upper limit and alower limit;

FIG. 8 shows a plot of the output power drawn from an ultracapacitorbank;

FIG. 9 shows a plot of the stored electrical energy in an ultracapacitorbank; and

FIG. 10 shows a flowchart of steps for limiting the input power betweenan upper limit and a lower limit.

DETAILED DESCRIPTION

Mining excavators include electric shovels and draglines. FIG. 1 shows aschematic of an electric shovel 100 to illustrate a mining excavatorthat consumes significant electrical power. The major components arecrawler 102, deck 104, boom 106, hoist 108, handle 110, and dipper 112.Electric motors enable various motions to operate the electric shovel100. Motion 131 propel (forward/reverse directions) refers to travel ofthe entire electric shovel 100 with respect to the ground. Motion 133swing (away/return directions) refers to rotation of deck 104 withrespect to crawler 102. Motion 135 crowd (crowd/retract directions)refers to positioning of dipper 112 with respect to boom 106. Motion 137hoist (hoist/lower directions) refers to positioning dipper 112 up anddown with respect to the ground. Multiple electric motors may be used toprovide each motion.

An electric shovel typically performs a series of repetitive operations.For example, it may propel forward near a bank, swing the dipper intoposition, crowd the dipper into the bank, hoist the dipper to scoop outmaterial, retract the dipper, propel in reverse to clear the bank,propel forward to a dump site, swing the dipper into position, lower thedipper, and dump the load. It then returns to the bank and repeats theoperation. Motors, then, often accelerate in one direction, brake, andaccelerate in the opposite direction. The mechanical load on a motor ishighly variable. As one example, consider a motor hoisting a dipper fullof heavy material, dumping the material, and lowering an empty bucket.

From an electrical power perspective, an electric shovel presents acyclic load to an electrical power source. As a function of operatingtime, the electrical power drawn by the electric shovel variescyclically. The variation in power may be significant: as previouslymentioned, the average power drawn by these machines may be about 55% oftheir peak power demand.

Under normal operation, an electric motor converts electrical energyinto mechanical energy. An electric motor may also be operated inreverse as a generator to convert mechanical energy into electricalenergy. Under normal operation, an electric motor draws (consumes)electrical power from an electrical power source. When an electric motorunder motion is stopped, the residual mechanical energy may be convertedto electrical energy. Herein, a time interval during which an electricalload is drawing electrical energy is referred to as a motoring interval;and a time interval during which an electrical load is generatingelectrical energy is referred to herein as a regeneration interval.

In FIG. 2A, electrical power source 202 feeds total user load 204. Inthis example, total user load 204 comprises application load 206.Controller 208 controls the electrical power transferred betweenelectrical power source 202 and application load 206. Electrical powerP₁ 221 represents the output electrical power from electrical powersource 202. Electrical power P₂ 223 represents the input electricalpower drawn by application load 206, which, in this example, is a cyclicload. The input electrical power required to operate a load is alsoreferred to as the electrical power demand of the load.

FIG. 2B shows plot 230 of the electrical power demand P₂ 223 (verticalaxis) of application load 206 as a function of time t (horizontal axis).In this example. P₂ ranges from zero to positive values. When theelectrical power demand is positive, application load 206 is drawingelectrical power. Plot 230 is a generic plot used for illustration. Theactual power demand is dependent on the specific equipment and operatingconditions. For simplicity, many portions of plot 230 are shown asstraight line segments. In general, the shape is arbitrary (for example,curvilinear).

In FIG. 2B, cycle 231-cycle 247 are examples of cycles. Note that thecycles are not necessarily strictly periodic. The functional dependenceof power vs. time, the amplitude, and the duration of each cycle mayvary. In addition to up/down variations, a cycle may include othergeometric features, such as a plateau (constant power) in cycle 237 anda cusp in cycle 241.

FIG. 2C shows the corresponding plot 240 of the output electrical powerP₁ 221 (vertical axis) from electrical power source 202. When the outputelectrical power is positive, electrical power is drawn from electricalpower source 202. In the example shown, the output electrical power P₁221 (plot 240 in FIG. 2C) is equal to the electrical power demand P₂ 223(plot 230 in FIG. 2B).

FIG. 2D shows a different example, plot 250, of the electrical powerdemand P₂ 223 (vertical axis) of application load 206 as a function oftime t (horizontal axis). Note that the power ranges from positive tonegative values. When the electrical power demand is positive (motoringregion), application load 206 is drawing electrical power. When theelectrical power demand is negative (regeneration region), applicationload 206 is generating electrical power. As shown in plot 250,application load 206 generates electrical power during time interval 251(t₁≦t≦t₂), time interval 253 (t₃≦t≦t₄), and time interval 255 (t₅≦t≦t₆).

FIG. 2E shows the corresponding plot 260 of the output electrical powerP₁ 221 (vertical axis) from electrical power source 202. When the outputelectrical power is positive, electrical power is drawn from electricalpower source 202. When the output electrical power is negative,electrical power is fed back into electrical power source 202. In theexample shown, in the motoring region, the output electrical power P₁221 (plot 260 in FIG. 2E) is equal to the electrical power demand P₂ 223(plot 250 in FIG. 2D). In the regeneration region (time interval 251,time interval 253, and time interval 255), the output electrical powerP₁ 221 is zero. In this example, the electrical power generated in theregeneration region is fed into a resistor (not shown) and converted towaste heat. The electrical power generated in the regeneration regionmay also be fed back to electrical power source 202. The outputelectrical power P₁ 221 would then be negative during time interval 251,time interval 253, and time interval 255.

FIG. 3A shows a schematic of an electrical power system, according to anembodiment of the invention, that recaptures the electrical powergenerated in the regeneration region. Electrical power source 302 feedstotal user load 304. In this example, total user load 304 comprisesapplication load 306 and electrical energy storage unit 310. Controller308 controls the electrical power transferred between electrical powersource 302 and application load 306, between electrical power source 302and electrical energy storage unit 310, and between application load 306and electrical energy storage unit 310. Electrical power P₁ 331represents the output electrical power from electrical power source 302.Electrical power P₂ 333 represents the input electrical power drawn byapplication load 306, which, in this example, is a cyclic load.Electrical power P₃ 335 represents the electrical power generated byapplication load 306 in the regeneration region. Electrical power P₄ 337represents the electrical power received by electrical energy storageunit 310 from application load 306. Electrical power P₅ 339 representsthe output electrical power from electrical energy storage unit 310.

An example of electrical energy storage unit 310 is an ultracapacitor,which is characterized by high power densities. For increased electricalenergy storage, multiple ultracapacitors may be connected in series andparallel to form an ultracapacitor bank. Electrical current flowing intoan ultracapacitor charges the ultracapacitor, and electrical energy isstored via charge separation at an electrode-electrolyte interface. Thestored electrical energy may then later be used to output an electricalcurrent. In FIG. 3A, electrical power P₃ 335 generated by applicationload 306 may be fed as electrical power P₄ 337 to charge electricalenergy storage unit 310. In addition, electrical power P₁ 331 output byelectrical power source 302 may be fed as electrical power P₅ 339 tocharge electrical energy storage unit 310.

FIG. 3B shows plot 390 of the electrical power demand P₂ 333 (verticalaxis) of application load 306 as a function of time t (horizontal axis).Note that, in this example, the power ranges from positive to negativevalues. When the electrical power demand is positive (motoring region),application load 306 is drawing electrical power. When the electricalpower demand is negative (regeneration region), application load 306 isgenerating electrical power. As shown in plot 390, application load 306generates electrical power during time interval 367 (t₁≦t≦t₂), timeinterval 369 (t₃≦t≦t₄), and time interval 371 (t₅≦t≦t₆). Since energy isthe integral of power over time, area 366, area 368, and area 370represent the electrical energy generated by application load 306 duringtime interval 367, time interval 369, and time interval 371,respectively. This electrical energy is stored in electrical energystorage unit 310.

In an embodiment of the invention, electrical power drawn fromelectrical energy storage unit 310 is used to reduce peak electricalpower drawn from electrical power source 302. FIG. 3C shows the plot 392of the output electrical power P₁ 331 (vertical axis) from electricalpower source 302. In this example, the lower limit of P₁ 331 is zero. Asdiscussed in examples below, the lower limit may also be greater thanzero or less than zero, depending on the cycle and the storage capacityof electrical energy storage unit 310. When the output electrical poweris positive, electrical power is drawn from electrical power source 302.When the output electrical power is negative, electrical power is fedback into electrical power source 302. In the example shown, in themotoring region, an upper limit P_(UL) 394 is placed on the outputelectrical power P₁ 331. For values of P₂≦P_(UL) (plot 390 in FIG. 3B),P₂ is supplied only by P₁ . For values of P₂>P_(UL), P₁ supplies a valueof P_(UL). The additional electrical power required is supplied by P₅339 drawn from electrical energy storage unit 310.

Referring to FIG. 3B, P₂ is greater than P_(UL) during time interval 361(T₁≦t≦T₂), time interval 363 (T₃≦t≦T₄), and time interval 365 (T₅≦t≦T₆).Note that the difference P₂−P_(UL) are represented by pulses, referencedas pulse 350, pulse 352, and pulse 354, respectively. The pulseamplitudes are referenced as amplitude 380, amplitude 382, and amplitude384, respectively. The energy drawn within each pulse is referenced aspulse energy 360, pulse energy 362, and pulse energy 364, respectively.As discussed above, energy is represented by area in a power vs. timeplot. In the example shown, the pulses have a triangular shape. Ingeneral, the pulse shape may vary, depending on the application load andoperating conditions and other factors.

In an embodiment of the invention, the electrical energy storage unit310 is configured such that it may supply all pulse energy requiredduring the operation of application load 306. Parameters to beconsidered in configuring the electrical energy storage unit 310 includepulse amplitude, pulse width, pulse shape, and time interval betweenpulses. If P₃ is not sufficient to maintain adequate charge inelectrical energy storage unit 310. P₁ may also be used during off-peakperiods to charge electrical energy storage unit 310.

In an embodiment of the invention, if application load 306 operates onlyin the motoring region (no regeneration), electrical energy storage unit310 may be charged entirely by P₁ 331 from electrical power source 302.The charging may occur during off-peak demand to limit the power P₁ 331from electrical power source 302 during peak demand. FIG. 3D shows plot3100 of the electrical power demand P₂ 333 (vertical axis) ofapplication load 306 as a function of time t (horizontal axis). In thisexample. P₂ ranges from zero to positive values. When the electricalpower demand is positive, application load 306 is drawing electricalpower.

FIG. 3E shows the plot 3200 of the output electrical power P₁ 331(vertical axis) from electrical power source 302. When the outputelectrical power is positive, electrical power is drawn from electricalpower source 302. In the example shown, an upper limit P_(UL) 3144 isplaced on the output electrical power P₁ 331. For values of P₂≦P_(UL)(plot 3100 in FIG. 3D), P₂ is supplied only by P₁. For values ofP₂>P_(UL), P₁ supplies a value of P_(UL). The additional electricalpower required is supplied by P₅ 339 drawn from electrical energystorage unit 310. In FIG. 3D, note that P₂>P_(UL) for pulse 3102-pulse3110. The corresponding pulse energies are pulse energy 3122-pulseenergy 3130, respectively, which are supplied by electrical energystorage unit 310. Note that a positive lower limit P_(LL) 3146 (FIG. 3E)may be placed on the output electrical power P₁ 331. Advantages ofmaintaining a lower limit are discussed below.

Electrical drive motors used in mining excavators typically operate on3-phase alternating current (AC). Mining excavators are typicallypowered from an electrical power distribution network feedinghigh-voltage AC power through high-voltage armored trail cables to theprimary side of a drive power transformer; more than one drive powertransformer may be used. A drive power transformer has multiplesecondary windings which supply power to a regenerative boost rectifiersystem through line reactors. Such a rectifier system may compriseactive front ends (AFEs). The active front ends are pulse-widthmodulated isolated gate bipolar transistor (IGBT) rectifiers thatconvert the incoming AC into direct current (DC) and store the energy inlow inductive DC link capacitors. A regenerative rectifier system mayalso be implemented with silicon-controlled rectifier (SCR) bridges. Thenumber of drive power transformers and the number of active front endsdepend on the total power requirement of the drive motors. Powerelectronic inverters invert the DC voltage available at the DC link toAC voltages that feed the motors.

Electrical energy regenerated by the motors may be fed back to thepublic utility electrical grid by the active front ends. Under a lineside fault condition, however, the active front end may not be able tosend back all the regenerated energy to the public utility electricalgrid, and the machine needs to shut down. The regenerated energy whichcould not be fed back to the public utility electrical grid may lead toan increase in the DC link voltage. Since high DC link voltages maydamage components, and also pose a safety hazard, protective circuitssuch as DC choppers and crowbars are added to the system to suppressexcessive DC link voltages during regeneration. In this instance,electrical energy is converted to waste heat. As discussed below, in anembodiment of the invention, regenerated electrical energy is stored inan ultracapacitor bank and used to supplement the power to the drivemotors during peak demand.

FIG. 4 shows a single-line diagram of electric shovel control system400. Block 401 represents input electrical power plant. Block 403represents electrical power converters. Block 405 represents electricaldrive motors.

Referring to block 405, electric shovel 100 is equipped with sixelectrical drive motors, referenced as motor 4100-motor 4110. Each motorruns on 3-phase alternating current (AC).

In the example shown in block 401, electrical power is directly fed froma public utility electrical power grid via substation 404 supplying3-phase AC power at a voltage of 3.3 or 6.6 kV. Substation 404 isconnected via switch 406 to current transducer 408 and switch 410. Poweris supplied via fuse 412 and switch 414 to the primary side of drivepower transformer 420. Similarly, power is supplied via fuse 422 andswitch 424 to the primary side of drive power transformer 426. Potentialtransducer 416 generates a synchronizing voltage feedback signal 418.

Referring to block 403, one output from the secondary side of drivepower transformer 420 is connected via current transducer 430 and activefront end (AFE) choke/reactor 438 to AFE AC-to-DC converter 446. Asecond output from the secondary side of drive power transformer 420 isconnected via current transducer 432 and AFE choke/reactor 440 to AFEAC-to-DC converter 448. Similarly, one output from the secondary side ofdrive power transformer 426 is connected via current transducer 434 andAFE choke/reactor 442 to AFE AC-to-DC converter 450. A second outputfrom the secondary side of drive power transformer 426 is connected viacurrent transducer 436 and AFE choke/reactor 444 to AFE AC-to-DCconverter 452. Output DC voltages are monitored by potential transducer454 and potential transducer 456.

DC power from the outputs of AFE AC-to-DC converter 446-AFE AC-to-DCconverter 452 is fed to the inputs of AFE DC-to-AC inverter 458-AFEDC-to-AC inverter 464. Block 480 represents a ground fault detectioncircuit. Block 482 represents an overvoltage chopper circuit thatdissipates excess electrical energy through resistor 484. The outputs ofAFE DC-to-AC inverter 458-AFE DC-to-AC inverter 464 are connectedthrough current transducer 466-current transducer 472, respectively, tomotor 4100-motor 4110. Switching drive power between different motorsmay be performed via transfer switch 490/492.

FIG. 5 shows a schematic of an ultracapacitor bank electrical energystorage unit integrated into an existing electrical power convertersystem. The existing electrical power converter system is represented bydrive power transformer 502, AFE choke/reactor 504, AFE choke/reactor506, AFE 508, AFE 510, DC link 512 and inverter 514. Motor 560represents a cyclic load. As previously shown in FIG. 4, inverter 514may feed more than one motor. The ultracapacitor electrical energystorage unit 540 comprises DC-to-DC converter 542/544, choke/reactor546, and ultracapacitor bank 548. The ultracapacitor electrical energystorage unit 540 may be disconnected from the electrical power convertersystem via disconnect switch 550. The ultracapacitor electrical energystorage unit 540 is managed by ultracapacitor energy managementcontroller 550.

Note that a mining excavator with a system of multiple electric motorsmay be viewed as a single unified cyclic load that operates duringmotoring intervals and regeneration intervals. The electric shovel 100in FIG. 1 appears as a cyclic load to the public utility electrical gridgoverned by a duty cycle. FIG. 6 shows an example of a power cycle forelectric shovel 100 over a 30-sec duty cycle. The horizontal axis 602represents time in seconds (sec). The vertical axis 604 represents powerin kilowatts (kW). Plot 606 represents the power demand of electricshovel 100.

In this example, note that on vertical axis 604, the power ranges frompositive values to negative values. For positive values, indicated bymotoring region 608, the electric shovel 100 is drawing power. Fornegative values, indicated by regeneration region 610, the electricshovel 100 is generating power. In the duty cycle shown in FIG. 6, thereare three time intervals during which electric shovel 100 operates inthe regeneration region 610: t₁ 641-t₂ 643, t₃ 645-t₄ 647, and t₅ 649-t₆651.

The maximum power demand presented by electric shovel 100 is P_(max)612. In the standard electrical power converter system, all power issupplied by drive power transformer 502 (FIG. 5). Therefore, in motoringregion 608, plot 606 also represents the power supplied by drive powertransformer 502. In an embodiment of the invention, the maximum powersupplied by drive power transformer 502 is set at a user-defined valueP_(UL) 616 (UL=Upper Limit). In an embodiment of the invention, duringthe time intervals in which peak demand exceeds P_(UL) 616, the powerexceeding P_(UL) 616 is supplied by the ultracapacitor bank 546. Oneskilled in the art may set the value of P_(UL) 616 according to specificequipment and applications.

FIG. 7 shows a modified power profile of drive power transformer 502.The horizontal axis 602 represents the 30-sec duty cycle previouslyshown in FIG. 6. The vertical axis 704 represents power in kilowatts(kW). Plot 706 represents the power delivered by drive power transformer502. Note that the power falls between P_(UL) 616 and P_(LL) 718(LL=Lower Limit). The lower limit P_(LL) 718 may be set to zero, anegative value, or a positive value. The lower limit is set to zero ifdrive power transformer 502 delivers zero power during a regenerationinterval, and all regenerated electrical energy is stored inultracapacitor bank 546. The lower limit is set to a negative value ifthe capacity of ultracapacitor bank 546 is not sufficient to store allof the regenerated electrical energy: a portion of the regeneratedelectrical energy is stored in ultracapacitor bank 546, and a portion ofthe regenerated electrical energy is returned to the public utilityelectrical grid. The lower limit is set to a positive value (as shown inthe example in FIG. 7) if the regenerated electrical energy is notsufficient to fully charge ultracapacitor bank 546: electrical powerfrom drive power transformer 502 is also used to charge ultracapacitorbank 546 during off-peak intervals. Note that line power ripple isdecreased as P_(LL) 718 is increased. It is therefore advantageous toset P_(LL) 718 as high as possible, consistent with the duty cycle andvoltage of electrical energy storage unit 310. One skilled in the artmay set the value of P_(LL) 718 according to specific equipment andapplications.

FIG. 8 shows the corresponding ultracapacitor power profile. Thehorizontal axis 602 represents the 30-sec duty cycle previously shown inFIG. 6. The vertical axis 804 represents power in kilowatts (kW). Plot806 represents the power profile of ultracapacitor bank 546 (FIG. 5).Note that on vertical axis 804, the power ranges from positive values tonegative values. For positive values, indicated by regeneration region808, the ultracapacitor bank 546 is drawing power (charging theultracapacitor bank). For negative values, indicated by motoring region810, the ultracapacitor bank 546 is generating power (discharging theultracapacitor bank).

FIG. 9 shows the energy stored in the ultracapacitor system. Thehorizontal axis 602 represents the 30-sec duty cycle previously shown inFIG. 6. The vertical axis 904 represents energy in kilojoules (kJ). Plot906 is calculated by integrating the power (represented by plot 806 inFIG. 8) as a function of time. This data is used for proper sizing ofultracapacitor bank 546. Once the ultracapacitor bank 546 has therequired energy stored, additional energy, if available, could be sentback to the public utility electrical grid. In one example, theoperating voltage of the ultracapacitor system is approximately 1400 to1800 volts, and the total capacitance of the ultracapacitor system isapproximately 4.5 to 9 farads. One skilled in the art may set designrequirements for the ultracapacitor system according to specificequipment and applications.

FIG. 10 is a flowchart summarizing steps for limiting the power drawnfrom an electrical power source by a cyclic load. In step 1002, theupper power limit P_(UL) and the lower power limit P_(LL) are set. Theprocess then passes to step 1004, in which the input power drawn by thecyclic load is measured. The process then passes to step 1006, in whichthe measured input power drawn by the cyclic load is compared to theupper and lower power limits of the electrical power source. If themeasured input power drawn by the cyclic load falls within the upper andlower power limits, then the process passes to step 1008, in whichnormal operation continues. If the measured input power drawn by thecyclic load does not fall within the upper and lower power limits, thenthe process passes to step 1010, in which the measured input power drawnby the cyclic load is compared to the upper power limit P_(UL) and thelower power limit P_(LL). If the measured input power drawn by thecyclic load is greater than the upper power limit P_(UL), the processthen passes to step 1014, in which the ultracapacitor bank suppliespower to the DC link. If the measured input power drawn by the cyclicload is less than the lower power limit P_(LL), the process then passesto step 1012, in which the ultracapacitor bank draws power from the DClink. As discussed above, the ultracapacitor bank may be charged fromthe electrical power source. If the cyclic load operates in both amotoring region and a regeneration region, the ultracapacitor bank mayalso be charged from electrical power generated by the cyclic load.

Upon completion of either step 1012 or step 1014, the process passes tostep 1016, in which the voltage across the ultracapacitor bank ismonitored. The process then returns to step 1002, in which the upperpower limit and the lower power limit are reset if needed.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention. Those skilled inthe art could implement various other feature combinations withoutdeparting from the scope and spirit of the invention.

1. A method for supplying electrical power to at least one miningexcavator, each mining excavator comprising at least one electricalmotor, the at least one mining excavator comprising a cyclic load, themethod comprising: receiving alternating-current electrical power froman electrical power grid; converting the alternating-current electricalpower to direct-current electrical power; feeding a direct-current linkwith the direct-current electrical power; charging an electrical energystorage unit with electrical power drawn from the direct-current link;supplying electrical power to the cyclic load from only thedirect-current link when the electrical power drawn by the cyclic loadis less than or equal to an upper limit; and supplying first electricalpower to the cyclic load from the direct-current link and secondelectrical power to the cyclic load from the electrical energy storageunit when the electrical power drawn by the cyclic load is greater thanthe upper limit, wherein the first electrical power is less than orequal to the upper limit.
 2. The method of claim 1, wherein theelectrical energy storage unit comprises at least one ultracapacitor. 3.The method of claim 1, wherein the sum of the electrical power drawnfrom the direct-current link by the cyclic load and the electrical powerdrawn from the direct-current link by the electrical energy storage unitis greater than or equal to a lower limit.
 4. The method of claim 1,wherein the electrical power grid comprises a public utility electricalpower grid.
 5. The method of claim 1, wherein the electrical power gridoutputs three-phase alternating current with a minimum voltage of atleast one kilovolt.
 6. The method of claim 5, wherein the minimumvoltage is at least three kilovolts.
 7. The method of claim 1, whereinthe at least one mining excavator comprises at least one of: an electricshovel; and a dragline.
 8. An electrical power system for supplyingelectrical power to at least one mining excavator, each mining excavatorcomprising at least one electrical motor, the at least one miningexcavator comprising a cyclic load drawing electrical power from theelectrical power system, the electrical power system comprising: anelectrical power conversion system configured to receivealternating-current electrical power from an electrical power grid andconvert the alternating-current electrical power to direct-currentelectrical power; a direct-current link configured to receive thedirect-current electrical power; an electrical energy storage unit; anda controller configured to: supply electrical power to the electricalenergy storage unit from the direct-current link; supply electricalpower to the cyclic load from only the direct-current link when theelectrical power drawn from the electrical power system by the cyclicload is less than or equal to an upper limit; and supply firstelectrical power to the cyclic load from the direct-current link andsecond electrical power to the cyclic load from the electrical energystorage unit when the electrical power drawn from the electrical systemby the cyclic load is greater than the upper limit, wherein the firstelectrical power is less than or equal to the upper limit.
 9. Theelectrical power system of claim 8, wherein the electrical energystorage unit comprises at least one ultracapacitor.
 10. The electricalpower system of claim 8, wherein the sum of the electrical powersupplied to the cyclic load from the direct-current link and theelectrical power supplied to the electrical energy storage unit from thedirect-current link is greater than or equal to a lower limit.
 11. Theelectrical power system of claim 8, wherein the electrical power gridcomprises a public utility electrical power grid.
 12. The electricalpower system of claim 8, wherein the electrical power grid outputsthree-phase alternating current with a minimum voltage of at least onekilovolt.
 13. The electrical power system of claim 12, wherein theminimum voltage is at least three kilovolts.
 14. The electrical powersystem of claim 8, wherein the at least one mining excavator comprisesat least one of: an electric shovel; and a dragline.
 15. A method forsupplying electrical power to at least one mining excavator, each miningexcavator comprising at least one electrical motor, the at least onemining excavator comprising a cyclic load drawing electrical powerduring at least one motoring interval and generating electrical powerduring at least one regeneration interval, the method comprising:receiving alternating-current electrical power from an electrical powergrid; converting the alternating-current electrical power todirect-current electrical power; feeding a direct-current link with thedirect-current electrical power; charging an electrical energy storageunit with the electrical power generated by the cyclic load during theat least one regeneration interval; supplying electrical power to thecyclic load from only the direct-current link when the electrical powerdrawn by the cyclic load is less than or equal to an upper limit; andsupplying first electrical power to the cyclic load from thedirect-current link and second electrical power to the cyclic load fromthe electrical energy storage unit when the electrical power drawn bythe cyclic load is greater than the upper limit, wherein the firstelectrical power is less than or equal to the upper limit.
 16. Themethod of claim 15, wherein the electrical energy storage unit comprisesat least one ultracapacitor.
 17. The method of claim 15, furthercomprising the step of: charging the electrical energy storage unit withelectrical power drawn from the direct-current link.
 18. The method ofclaim 17, wherein the sum of the electrical power drawn from thedirect-current link by the cyclic load and the electrical power drawnfrom the direct-current link by the electrical energy storage unit isgreater than or equal to a lower limit.
 19. The method of claim 15,wherein the electrical power grid comprises a public utility electricalpower grid.
 20. The method of claim 15, wherein the electrical powergrid outputs three-phase alternating current with a minimum voltage ofat least one kilovolt.
 21. The method of claim 20, wherein the minimumvoltage is at least three kilovolts.
 22. The method of claim 15, whereinthe at least one mining excavator comprises at least one of: an electricshovel; and a dragline.
 23. An electrical power system for supplyingelectrical power to at least one mining excavator, each mining excavatorcomprising at least one electrical motor, the at least one miningexcavator comprising a cyclic load drawing electrical power from theelectrical power system during at least one motoring interval andgenerating electrical power during at least one regeneration interval,the electrical power system comprising: an electrical power conversionsystem configured to receive alternating-current electrical power froman electrical power grid and convert the alternating-current electricalpower to direct-current electrical power; a direct-current linkconfigured to receive the direct-current electrical power; an electricalenergy storage unit configured to receive the electrical power generatedby the cyclic load during the at least one regeneration interval; and acontroller configured to: supply electrical power to the cyclic loadfrom only the the direct-current link when the electrical power drawnfrom the electrical power system by the cyclic load is less than orequal to an upper limit; and supply first electrical power to the cyclicload from the direct-current link and second electrical power to thecyclic load from the electrical energy storage unit when the electricalpower drawn from the electrical system by the cyclic load is greaterthan the upper limit, wherein the first electrical power is less than orequal to the upper limit.
 24. The electrical power system of claim 23,wherein the electrical energy storage unit comprises at least oneultracapacitor.
 25. The electrical power system of claim 23, wherein theelectrical energy storage unit is further configured to receiveelectrical power from the direct-current link.
 26. The electrical powersystem of claim 25, wherein the sum of the electrical power drawn by thecyclic load from the direct-current link and the electrical power drawnby the electrical energy storage unit from the direct-current link isgreater than or equal to a lower limit.
 27. The electrical power systemof claim 23, wherein the electrical power grid comprises a publicutility electrical power grid.
 28. The electrical power system of claim23, wherein the electrical power grid outputs three-phase alternatingcurrent with a minimum voltage of at least one kilovolt.
 29. Theelectrical power system of claim 28, wherein the minimum voltage is atleast three kilovolts.
 30. The electrical power system of claim 23,wherein the at least one mining machine comprises at least one of: anelectric shovel; and a dragline.